1. Field of the Invention
Our invention relates to a clarification tank such as a flotation cell, a settling tank, or a holding tank, wherein a mixture of two or more liquid materials having different densities is allowed to settle to effect a gravity separation and the clarified liquid is removed from the bottom of the tank through a central collection plenum which is connected to an external riser.
There are many industrial uses for clarification tanks wherein a mixture of two or more liquids having different specific gravities are separated by what is basically a gravity settling process, wherein the lighter liquid material collects at the top of the liquid mixture and the heavier material settles to the bottom of the container housing the mixture. For example, such clarification techniques are used in the mining, metallurgical and chemical processing arts, and in the oil industry -- both in refinery operations and crude oil production. Our present invention involves clarification tanks which are useful in each of the above operations, however, it is described herein with respect to crude oil production and waste water treatment required in production operations. In oil field water flooding operations, which are commonly used as a secondary recovery method for crude oil production, oil is produced by injection of water into an oil bearing sand formation to displace the oil and water in the sand formation towards adjacent wells through which it is brought to the surface. The oil thus produced is an oil-water mixture containing about 60% to 95% water, and the oil in such production mixtures is normally separated from the water by physical separation means, such as by settling tanks with the aid of emulsion breaking chemicals. The water separated from this mixture still contains on the order of 50 to 500 ppm oil. Consequently, this waste water is subjected to further clean-up operations to remove the residual 50-500 ppm oil prior to final disposal of the waste water or reuse in an industral application. According to conventional practices, the residual small amount of oil is removed from the waste water in a flotation cell operation.
2. Description of the Prior Art
The separation of small amounts of oil from water in a flotation cell is accomplished by injecting air into the oil-water mixture under pressure and pumping the mixture into a retention tank and then introducing the pressurized mixture into the flotation cell at a point beneath the surface of the liquid. Such processing is described more fully in U.S. Pat. Nos. 3,576,738 and 3,725,264. As the mixture is released into the body of liquid in the flotation cell, the air in solution is released and bubbles through the body of the liquid in the cell and attaches to suspended oil particles which effectively reduces their specific gravity and causes the oil to collect near the surface of the liquid in the cell or tank where it can be skimmed off with a skimmer into a skimming trough.
The liquid level in a flotation cell is quite important and is controlled by manipulation of adjustable rings or weirs at the top of the water outlet risers to maintain the water level. According to the prior art, several water removal risers are spaced circumferentially around the tank. Hence the adjustment of the weirs at the top of the several risers must be done evenly as the water level is maintained, in order to prevent short circuiting or by-passing, that is, passage of the oil-water mixture directly from the inlet tube to one of the risers. The multiple riser system is difficult to adjust properly. The by-passing phenomena may occur to a lesser or greater degree, and has the effect of reducing the effective residence time in the flotation cell thus decreasing the quality of the water recovered from the cell since the air bubbles are not given sufficient opportunity to remove the residual oil by attaching to the oil particles and floating these oil particles to the surface. In addition, by-passing increases turbulence in the flotation cell, which also reduces oil removal capability.
Some of the prior art tank designs recognize the advantages in removing the clarified liquid at multiple points in the tank; however, the prior art systems using multiple pick-up points did not provide for equalization of the flow rate through the multiple pick-up conduits and thus did not obviate the operational problems discussed above, i.e. channeling and by-passing, which problems are inherent in the use of multiple pick-up conduits through which the liquid is removed at different flow rates.
The risers in prior art flotation cells, typically, are open to the inside of the tank and this arrangement increases the likelihood of by-passing.
Accurate liquid level control is also essential since too high a level will pass water with the skimmed oil. Liquid level control is accomplished in the prior art by removing adjustable rings in the top of water removal risers, as mentioned above or by using compartmented weir boxes such as those shown in U.S. Pat. No. 3,246,763. Compartment weir boxes are limited in the volume of water which can be removed with a given size weir box since water is removed effectively from only one opening.
According to the prior art, sand and sediment carried into the flotation cell with the oil-water mixture to be separated, are removed from the flotation cells by providing a conical bottom surface in the cell or tank and a rotary sweep to move the sand and sediment to an outlet for removal.
According to the present invention, sand and sediment are removed from the tank by providing a series of sand pan draw-offs through which sand and sediment are removed to a central collection line hydrostatic pressure differential.